Blood oxygen saturation (SO2) is a measure of the amount of oxygen dissolved or carried in the blood. More specifically, blood SO2 represents the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. State-of-the-art pacemakers and ICDs are typically equipped with one or more sensors for detecting blood SO2, as an accurate measure of blood SO2 is very useful for a variety of diagnostic purposes. For example, blood SO2 is useful for tracking cardiac output within the patient, i.e. the amount of blood pumped by the heart per minute. See, e.g., U.S. Patent Application 2005/0245833 of Kline entitled “Non-Invasive Device and Method for Measuring the Cardiac Output of a Patient.” Blood SO2 is also used to evaluate autonomic tone. See, e.g., U.S. Pat. No. 6,942,622 to Turcott, “Method for Monitoring Autonomic Tone.” Blood SO2 is useful for controlling pacing therapy so as to achieve hemodynamically optimal therapy. See, e.g., U.S. Pat. No. 5,891,176 to Bornzin, entitled “System and Method for Providing Hemodynamically Optimal Pacing.” Blood SO2 may also be employed as an indictor of physical activity within a patient. See, e.g., U.S. Pat. No. 5,076,271 to Lekholm, et al., entitled “Rate-Responsive Pacing Method and System Employing Minimum Blood Oxygen Saturation as a Control Parameter and as a Physical Activity Indicator.”
Blood SO2 is also advantageously employed in connection with detecting various respiratory conditions such as apnea or periodic breathing. See, e.g., U.S. patent application Ser. No. 10/795,009, of Koh, entitled “System and Method for Distinguishing among Obstructive Sleep Apnea, Central Sleep Apnea and Normal Sleep Using an Implantable Medical System,” filed Mar. 4, 2004. Blood SO2 may also be used as one factor in evaluating heart failure and associated mortality. See, e.g., U.S. Pat. No. 6,645,153, to Kroll et al., entitled “System and Method for Evaluating Risk of Mortality Due To Congestive Heart Failure Using Physiologic Sensors” and U.S. Pat. No. 6,589,188 of Street, et al., “Method for Monitoring Heart Failure via Respiratory Patterns”. Depending upon the particular application, either arterial SO2 (i.e. SaO2), or venous SO2 (i.e. SvO2), or both, may be detected and exploited.
Unfortunately, blood SO2 is a difficult parameter for a pacemaker or other implantable medical device to accurately and reliably detect. To detect blood SO2, pacemakers typically employ an implanted optical sensor that includes a light emitting diode (LED), which transmits light into blood passing the sensor, and a phototransistor that senses the light after it has passed through the blood. Blood SO2 is then derived from a comparison of the intensity and frequency of the emitted light and the received light. In particular, pulse oximetry techniques are often employed by the pacemaker to determine the blood SO2 level from the light signals. See, e.g., U.S. Pat. No. 5,676,141 to Hollub, entitled “Electronic Processor for Pulse Oximeters.” Depending upon the implementation, an optical measurement window of the phototransistor of the sensor is either positioned within the blood stream (such as within one of the chambers of the heart) or is positioned subcutaneously near a blood vessel. If the measurement window is positioned in the blood stream, blood cells tend to fixate on the optical measurement window thus interfering with the sensor. If the measurement window is instead is mounted subcutaneously, then skin and/or muscle cells can grow over the window, likewise interfering with the sensor. Moreover, the intensity of the LED tends to decrease over its life time.
Hence, implantable blood SO2 sensors should be periodically calibrated to compensate for these and other factors. Calibration may be performed in conjunction with an external detector. The blood SO2 level within the patient is simultaneously detected using both the implanted sensor and the external detector. If the two values differ, a calibration factor is calculated to adjust the internally detected saturation value to match the externally detected value. The calibration is then repeated periodically, perhaps every several months or so, in an attempt compensate for blood cell fixation, tissue overgrowth, and other factors. Unfortunately, a significant amount of blood cell fixation or tissue overgrowth can occur in the interim, greatly affecting the output values of the sensor such that the pacemaker no longer receives correct saturation values, which may in turn lead to pacing that is counter-productive and perhaps even dangerous for the patient.
As can be appreciated, it would be far more desirable to provide a self-calibration technique that can be performed by the implanted system itself without need for a simultaneously-detected external oxygen saturation measurement. Such a calibration technique would allow the pacemaker to frequently adjust the calibration factors so as to promptly compensate for changes in blood cell fixation, tissue overgrowth, etc. One possible self calibration technique involves comparing the latest output signals from the sensor with previously detected and stored baseline signals. Any difference between the new output signals and the baseline signals is then attributed to changes in blood cell fixation, tissue overgrowth, etc., and appropriate calibration factors are calculated. A fundamental problem with the proposed technique is that it assumes that the actual blood SO2 level within the patient is the same for the newly sensed signals and for the baseline signals. If not, then any difference between the newly sensed signals and the baseline signals may be due to a difference in oxygen saturation instead of a difference in blood cell fixation or tissue overgrowth. In other words, the proposed technique, without further inventive modifications, cannot distinguish between variations in the sensed signals arising due to changes in oxygen saturation and variations due to other factors and hence is unable to properly calibrate the sensor.
Accordingly, it would be highly desirable to provide an effective self calibration technique for use with an implantable blood SO2 sensor and it is to this end that the invention is primarily directed.